This section is intended to provide a background or context to the invention that is recited in the claims. The description herein may include concepts that could be pursued, but are not necessarily ones that have been previously conceived, implemented or described. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and claims in this application and is not admitted to be prior art by inclusion in this section.
The following abbreviations are utilized herein:    ADC analog-digital conversion    CMOS complementary metal-oxide-semiconductor    CRA chief ray angle    DIS digital image stabilization    EIS electronic image stabilization    HDR high dynamic range    LSC lens shading correction    OIS optical image stabilization/optical image stabilizer
With improvements in miniaturization, it has become common for portable electronic devices, such as mobile phones and personal digital assistants (PDAs), to include a camera for still and/or motion photography. While providing additional functionality, the portable nature of the devices and smaller size of the components lead to further issues to be addressed.
Image sensors typically receive light incident on a two-dimensional array of photosensitive pixels. Each pixel may include a number of elements including, as non-limiting examples: a lens (often called a “microlens”), a color filter that is configured to block light of one or more particular color(s) (wavelengths) from reaching the sensor, and a photosensitive element. As an example, these elements may be arranged at different physical levels relative to, of or on a substrate. Traditionally, the elements of the pixels should have their centers substantially aligned (e.g., substantially coaxial). Reference is made to US Patent Application Publication No. 2007/0030379.
FIG. 1 shows a diagram illustrating CRA with respect to various image sensor components. Light from an object 110 is received by the device via a lens 112 and an aperture 114 in relation thereto. The light is incident on a sensor array 116 comprised of a two-dimensional array of photosensitive pixels. The sensor array 116 outputs a digital readout corresponding to the incident light as sensed by the individual pixels. The CRA is the angle of the incident rays passing through a center of the lens aperture as measured (e.g., at the image sensor) from the normal. Thus, the CRA of the incident light at a pixel corresponds to the angle of the rays as incident on the sensor array 116. As may be appreciated from FIG. 1, rays incident on the center of the sensor array 116 have a CRA of 0° (i.e., the normal; aligned with the optical axis shown in FIG. 1). The CRA typically increases for pixels that are further away from the center of the sensor array 116 with those pixels furthest from the center (e.g., at the edges of the sensor array 116) having the largest CRA. For example, CR1 118 is greater than CR2 120 and, thus, CR1 strikes a portion (e.g., a pixel) of the sensor array 116 further from the center than the portion that CR2 120 strikes.
Conventional CMOS pixels act as “photon buckets” that collect light towards the light-sensitive region at the bottom. As the CRA increases for a pixel, the light may be blocked by metal traces (e.g., along the walls—the sides of the “photon bucket”) which results in reduced light collection efficiency. FIG. 2 shows examples of the collection of light for different CRAs, namely for a CRA of 0° (FIG. 2A) and a CRA of 25° (FIGS. 2B and 2C). The light is received by a microlens 130 that focuses the received light and allows it to be sensed by a light sensitive region 132. Along the sides are metal traces 134. As can be seen, for a larger CRA (e.g., a CRA of 25° as in FIG. 2B) the metal traces 134 may block some of the received light from reaching the light sensitive region 132. Thus, it is often desirable to mitigate the effect of CRA when capturing images.
It is further noted that the size of a pixel may also affect the light collection and sensing. Generally, larger pixels are less susceptible than smaller pixels to pixel efficiency reduction at higher CRAs. For example, compare the larger pixel (at a CRA of 25°) of FIG. 2C to the smaller pixel (at a CRA of 25°) of FIG. 2B. As can be seen, more light is collected (e.g., sensed) by the larger pixel since the metal traces of the wider pixel block fewer rays.
Some sensor technologies such as a back side illumination sensor, for example, may be less prone to shading caused by a large CRA. To achieve optimum image quality, microlenses and optics may be optimized so that even a large CRA can produce good image quality. Even in such cases, a larger CRA may produce more severe lens shading problems.
Another issue with portable image capturing devices (e.g., cameras, mobile devices having camera functionality) is image blurring, for example, caused by vibration. One set of techniques that may be used to combat such blurring effects are image stabilization techniques. Examples of image stabilization techniques include DIS, EIS and OIS. An OIS component detects movement and varies the optical path through the camera lens to the image sensor to reduce the blurring effect on the image. Reference in this regard may be made to EP 1 729 509 and to commonly assigned U.S. patent application Ser. No. 11/267,394 (filed Nov. 4, 2005, published as US 2007/0103555) and Ser. No. 12/080,695 (filed Apr. 3, 2008, published as US 2009/0252488).
By way of further example, descriptions are provided below for various exemplary OIS technologies.                Lens System Shift: All lenses in the camera lens system are shifted in relation to the image sensor in order to vary the optical path.        Lens Shift or Partial Lens System Correction: As compared to the lens system shift noted above, not all of the lenses in the camera lens system are shifted. Instead, there is at least one correction lens that is shifted in order to vary the optical path. In some cases, the at least one correction lens may be such that its optical power is changed in order to vary the optical path.        Lens System Tilt, Lens Tilt or Variable Prism: The lens or lens system (i.e., one or more lenses) is tilted in relation to the image sensor in order to vary the optical path. The tilting effect also can be obtained by using a variable prism in the lens system.        Sensor Shift: The image sensor is shifted in relation to the lens in order to vary the optical path.        
Another issue that arises in the operation of image sensors is lens shading. Various imperfections in the optical assemble may create a shading effect on the produced image. For example, the image may be brighter in the center and decrease in brightness when approaching the edges of the sensor array. Generally, there may be a two-dimensional variation in shading across the sensor array caused by the optical elements in the assembly and the difficulties encountered in uniformly projecting an image across the field of view of the sensor array (e.g., CRA variations). Overall, shading may be caused by differences of an optimal optical path object through the optics/microlenses to the sensor as compared to the actual optical path. Various techniques may be used to correct the lens shading of the captured image, including the use of correction algorithms that apply a two-dimensional correction function in order to compensate for lens shading differences. Reference in this regard may be made to US Patent Application Publication No. 2009/0268053.